The present invention relates to a fine powder of epoxy-containing silicone elastomer and a method of manufacturing the same. More particularly, for example, the invention relates to a fine powder of epoxy-containing silicone elastomer which does not undergo self-welding, is highly miscible with different kinds of organic resins, and effectively modifies such organic resins. The invention also concerns a method of making such powder which involves formation of a silicone elastomer emulsion and separation of fine elastomer powder therefrom.
Several proposals have been made for methods for manufacturing particulate or granular silicone elastomers. For example, U.S. Pat. No. 3,843,601 discloses a method wherein hardened silicone elastomer is cooled at a temperature below the glass transition temperature and then pulverized. Japanese Patent Kokai No. 59-96122 (1984) discloses a method wherein an addition reaction-type hardenable organosiloxane compound is hardened by means of a spray dryer at a temperature of 230.degree.-300.degree. C. to thereby obtain particulate silicone elastomer. Japanese Patent Kokai No. 62-270660 (1987) discloses a method wherein an addition reaction-type or condensation-type hardenable organopolysiloxane compound is self-mixed under a low shearing stress whereby the compound is hardened and granulated. In these proposed methods it was difficult to obtain fine and regularly-sized particles, a specially designed apparatus was required, and control of the hardening rate was difficult. Due to these drawbacks, these methods were not considered industrially advantageous.
Japanese Patent Kokai No. 56-36546 (1981) teaches a method which comprises emulsifying an organosiloxane compound consisting of an organopolysiloxane having terminal vinyl group(s) and an organohydrogenpolysiloxane having Si-bonded hydrogen atom(s) with water and a surface active agent, and heating and thus hardening the emulsion after it is mixed with a Pt-containing catalyst to thereby obtain an emulsion containing silicone elastomer powder. However, this literature does not describe separation of the hardened silicone elastomer in particulate form.
Various methods have been proposed for separating the particles of silicone elastomer existing in water in the form of a dispersion or emulsion. (ref. Japanese Patent Kokai Nos. 62-243621 (1987), 62-257939 (1987), 63-77942 (1988), 64-56735 (1989), 64-70558 (1989), Japanese Patent Kokoku 63-65692 (1988)). According to these methods, it is possible to control the particle size through control of the emulsion preparation conditions and selection of emulsifier. Also, it is possible to control the degree of refining by washing the emulsion in a continuous process to remove impurities. However, in these methods, since the particles formed in the emulsion are so fine, separation is difficult.
Japanese Patent Kokai Nos. 62-243621 and 63-77942 disclose a method wherein a hardenable liquid silicone rubber compound is combined with a surface active agent at a temperature below 25.degree. C. where the compound does not harden to thereby produce an aqueous emulsion. The emulsion is then dispersed in water of a temperature higher than 25.degree. C. to allow formation of particulate rubber. In this method, control of the hardening reaction was difficult; thus, the particles merge with each other while hardened and form rubber particles. Consequently, it was impossible to separate sufficiently fine particles.
Incidentally, a method is known for destroying an emulsion, according to which a salt or an alcohol is added to the emulsion to thereby deactivate the surface active agent holding the emulsion particles. When this method is applied to an emulsion of a silicone elastomer, the deactivation is accompanied by condensation and merging of particles so that it is difficult to control the particle size and to effectively obtain well separated particles (ref. Japanese Patent Kokoku No. 62-257939).
As for the applications of silicone elastomer compounds in the form of or containing hardened silicone elastomer particles, there are several. For example, Japanese Patent Kokoku Nos. 62-28971 (1987) and 63-12489 (1988), Japanese Patent Kokai Nos. 55-3412 (1980), 61-101519 (1986), 61-166823 (1986), and 61-225253 (1986) discuss thermally hardenable epoxy resin compounds which are obtained by adding to an epoxy resin a silicone elastomer compound containing spherical particles of organopolysiloxane elastomer which is hardened through an addition or condensation reaction in the presence of Pt-containing catalyst, or heating in the presence of organic peroxide. In applications wherein a fine powder of such silicone elastomer is used as an agent to improve shock resistance or lubricity of thermoplastic resins or thermosetting resins, introduction of a functional group such as an epoxy group into this fine powder compound strengthens its miscibility and adhesion tendency with respect to different organic resins.
In fact, a number of silicone oils are known in which a functional group such as an epoxy group is introduced. Introduction of such a functional group in silicone elastomer fine powders was carried out, for example, in Japanese Patent Kokai Nos. 64-70558 (1989) and 64-56735 (1989). In these disclosures, a liquid silicone elastomer compound consisting of (i) an organopolysiloxane containing hydroxyl or alkenyl groups bonded to a silicon atom, (ii) an organohydrogenpolysiloxane containing hydrogen atoms bonded to a silicone atom, (iii) a catalyst for hardening, and (iv) an unsaturated hydrocarbon group-containing epoxy compound or an alkoxyl silane containing a functional group such as epoxy, is dispersed and hardened in water, and stripped of water by a spray dryer or the like. Silicone elastomer particles are thus obtained. However, in the silicone elastomer particles manufactured by this method, only part of the epoxy-containing compound is bonded to the elastomer molecule, while most of the compound is merely mixed in the silicone elastomer particles in unreacted form. For this reason, the physical properties of the elastomer particles are poor and the miscibility with different kinds of organic resins attributable to inclusion of a functional group cannot be sufficient attained.
In a method wherein a cross-linked rubber compound is hardened through condensation, some of the hydroxyl groups bonded to silicon atoms and some of the cleaved epoxy groups tend to undergo condensation prompted by a condensation catalyst so that the epoxy groups in the elastomer behave as crosslinking groups to thereby cause gel hardening. In methods wherein a cross-linked rubber compound is hardened through an addition reaction, it is difficult to control the hardening since the addition and crosslinking of epoxy-containing unsaturated compounds take place simultaneously.
Furthermore, the inventors proposed in Japanese Patent Kokoku No. 1-308187 (1989) a method for manufacturing a fine powder of epoxy-containing silicone elastomer made from a hardenable organopolysiloxane compound mainly consisting of (i) an organopolysiloxane containing in a molecule at least two vinyl groups bonded to silicon atoms and at least one epoxy-bearing organic group bonded to a silicon atom, (ii) an organohydrogenpolysiloxane containing in the molecule at least two hydrogen atoms bonded to silicon and (iii) a Pt-containing catalyst. According to this method, although a desirable fine powder of silicone elastomer is obtained, the preparation of the organopolysiloxane (i) requires two steps, namely an addition reaction and a rearrangement equilibration reaction in the presence of a basic compound, which renders the operation complicated and costly.